Those who drive vehicles on low traction surfaces such as roads covered with ice, snow, rain or loose gravel often have trouble controlling the behavior of the vehicle during turning or braking maneuvers because of the low coefficient of friction of such surfaces which substantially reduces the traction between the wheels and the road surface. As a result, application of the brakes or turning of the steering wheel can cause some or all of the wheels to skid. If the skid becomes severe, the vehicle can be difficult to control.
In recent years, anti-lock braking systems which operate during driver initiated braking to modulate the application of a braking force to all or some of the wheels of a vehicle to prevent wheel lockup have seen increased use. Other systems are known which operate during driver initiated braking to modulate the braking force applied independently of driver initiated braking to some or all of the wheels of a vehicle to improve vehicle stability while braking in a turn. Certain aspects of such a system are described by Shinji Matsumoto et al. in SAE Technical Paper Series No. 920645 entitled Improvement of Vehicle Dynamics Through Braking Force Distribution Control.
Anti-lock braking systems and systems of the type described by Matsumoto et al. operate during driver initiated braking to control vehicle stability. Other systems operate exclusively either in a torque control region or in a spin/skid control region to control vehicle stability. The applicants have discovered that on low traction surfaces, a selective combination of spin/skid control and torque control can lead to improved vehicle traction and stability.